This invention is directed to high density semiconductor memories, and more particularly, to a high density read-only-memory device which uses punch-through devices as the memory cells.
Read-only-memory devices are smaller in size than random access memories for a given number of bits of information. Generally, a read-only-memory cell consists of a single transistor, either metal-oxide-semiconductor or bipolar depending on the particular technology. A single transistor device gives a very small memory element. Single bipolar transistor memory cells are described in, "Design Consideration for a High-Speed Bipolar Read Only Memory" by Barrett et al in IEEE Journal of Solid State Circuits, pp. 196-202, October, 1970. Even smaller read-only-memories have been developed which utilize Schottkey diodes. Such a device is shown in "A Bipolar 16K ROM Utilizing Schottkey Diode Cells" by Gunn and Pritchett, IEEE International Solid-State Circuits Conference Digest of Technical Papers, pp. 118-119, February, 1977. A new type of memory cell which is even smaller than the Schottkey diode cell is the punch-through memory cell. This cell conducts current when a voltage is applied between adjacent impurity regions such that the depletion region from one of the impurity regions reaches the other impurity region. Such a device has been described in "Punch-Through Cell for Dense Bipolar ROMs" by Lohstroh and Slob, IEEE International Solid-State Circuits Conference Digest of Technical Papers, pp. 20-21, February, 1978. The punch through device is very fast because current flow is by drift rather by diffusion. The cell of Lohstroh in Slob is still larger than necessary because of the existence of contacts in the memory array.